1. Field of the Invention
This invention relates to a reversible thermochromic composition. More particularly, it relates to a reversible thermochromic composition capable of reversibly causing a color-developed state and a color-extinguished state depending on temperature changes. Still more particularly, it relates to a reversible thermochromic composition having a reversible metachromatic function that it presents a color-developed state when heated in a color-extinguished state and presents the color-extinguished state when temperature-dropped or cooled in the color-developed state, or having a reversible metachromatic function that it begins to develop a color in the course of temperature drop after heating, having been triggered by its temperature rise when heated in a color-extinguished state, and returns to the color-extinguished state after it has presented a color-developed state in a maximum developed-color density.
2. Description of the Related Art
With regard to reversible thermochromic compositions capable of causing reversible metachromatism in a specific temperature range upon electron-donating/accepting reaction, some proposals are disclosed in, e.g., U.S. Pat. Nos. 4,028,118, No. 4,720,301 and No. 4,732,810.
The proposals concerning conventional reversible thermochromic compositions are chiefly made on those having the function to exhibit reversible thermochromic properties such that they begin to become color-extinguished in the course of temperature rise caused in a color-developed state, present a completely color-extinguished state at a specific temperature or above, begin to develop a color in the course of subsequent temperature drop and return to the color-developed state (see FIG. 4), which are applied in temperature indicators, toys and so forth.
Meanwhile, an attempt to cause a color-developed state when heated in a color-extinguished state is disclosed in U.S. Pat. No. 4,421,560. This proposes a composition that utilizes temperature dependence of acid dissociation properties of an acidic phosphate compound, which, although exhibits a metachromatic behavior that presents the color-developed state when heated in a color-extinguished state, has an insufficient developed-color density. Also, such a metachromatic composition containing the acidic phosphate compound can be encapsulated in the form of microcapsules with difficulty, and moreover may hardly satisfy durability.
Some proposals on a high-temperature color-developing type are also disclosed in relation to reversible heat-sensitive recording materials in the field of heat-sensitive recording materials. Such reversible heat-sensitive recording materials are so made up that a color-developed state can be exhibited by applying a heat while keeping a thermal head of a thermal printer at a temperature exceeding hundred and tens of degrees. Hence, no effective metachromatic behavior can be presented within the life-environment temperature range, and there is a restriction on their application in the fields of temperature indicators, toys and so forth.